The present invention concerns a device for utilizing the thermal energy stored in soil to assist in the growth of a variety of plants and to protect the plants against frost and other cold weather conditions. Particularly, the present invention concerns a device and process in which a heat exchange device removes heat from the soil, in the vicinity of the roots of the plants, and transfers the extracted heat to the plants above the surface of the ground. This transfer of heat produces a number of beneficial effects for the plant, both as regards its root system and as regards the portion above ground.
Heat removal from the soil according to the present invention provides, with respect to one aspect of the invention, protection of the plants from frost and other cold weather conditions. Heretofore, a variety of means have been used to achieve this end. For example, fuel-burning individual heaters have been utilized in orchards in close proximity to the trees. This so-called "smudge pot" method has many advantages including smoke pollution, slow response time, significantly high fuel costs, and an inability to place the heaters in close proximity to the trees or plants to be protected. Much of the heat produced is therefore lost to the open sky. Additionally, fans have been utilized to blow either heater-created warm air or warm air from the higher strata of the atmosphere onto the trees. This method has also been unsuccessful because the fans have been effective in only limited areas. Conventional methods of orchard heating using the most efficient oil burners (return stack heaters) require from 1 million to 10 million BTU/hr/acre, even when augmented with wind machines which return some of the hot gases back down to the tree level. Such methods require fuel costs of at least about $50/acre/hr. Accordingly, they are seldom used in ares where the risk of freezing is low, e.g., in Texas.
Another method for protecting orchards is described in U.S. Pat. No. 2,350,621. According to this method, a plurality of pipes are embedded in the soil with a stem portion extending above-ground in the region of the plants or trees to be heated. A heating medium is provided which is circulated through the pipes. U.S. Pat. No. 4,216,614 also describes a system of conduits embedded in the ground, through which a heating medium is passed and by which heat is radiated to the atmosphere. Like the previous method, the latter methods require a significant energy expenditure for operation, but moreover, they require an enormous and generally prohibitive capital investment.
Increasing fuel costs, especially recently, have directed attention to providing processes for protecting agricultural crops which utilize other means of energy. One of these alternative energy supplies is geothermal energy. U.S. Pats. No. 3,470,943 and No. 3,521,699 describe the utilization of geothermal energy from deep within the earth for warming the ground in areas of severe cold. The geothermal heat is pumped to near the surface of the ground, after which it is utilized in conduit systems which have been embedded in the soil. This process, however, also involves a complex and prohibitively expensive conduit system which must extend to significant depths.
As an alternative to providing heat to the above-ground portion of the plant to prevent damage due to frost, suggestions have been made to extend the dormancy period of trees to avoid early frosts. U.S. Pat. No. 3,890,740 describes a method for retarding fruit budding by circulating a cooling fluid around the trunks of trees to maintain the interior temperature below 38.degree. F. and to control tree hydration and bud development. Such proposals likewise involve prohibitive levels of capital investment and high operating costs.
The present invention provides a device and method which achieve both of the above objectives, i.e., protection of plants against cold weather damage by means of a system requiring reasonable capital investment and low operating costs. Other advantages are achieved as well.